In-line smoke attenuator

ABSTRACT

In one form the present invention provides an apparatus in an airflow path before a particle detector, wherein the apparatus removes a substantially constant proportion of all sizes of airborne particles from the airflow over time. In an example the apparatus includes a flow splitting arrangement configured to divide a fluid flow into a plurality of sub-flows, the splitting arrangement  10  including means for defining a plurality of substantially identically dimensioned flow apertures configured to direct a portion of the fluid into a respective sub-flow.

RELATED APPLICATIONS

This application is a nationalization under 35 U.S.C. 371 ofPCT/AU2007/000189, filed Feb. 20, 2007 and published as WO 2007/095675A1 on Aug. 30, 2007, which claimed priority under 35 U.S.C. 119 toAustralian Patent Application Serial No. 2006900823, filed Feb. 20,2006; which applications and publication are incorporated herein byreference and made a part hereof.

FIELD OF INVENTION

The present invention relates to an improved method and apparatus forparticle detection. In a preferred form, the present invention relatesto a method and apparatus for filtering an air sample before applying itto a particle detector.

It will be convenient to describe the invention as applied to smokedetection, however the invention should not be construed as beinglimited to this exemplary field of use.

BACKGROUND OF THE INVENTION

Particle detectors are often used to warn of the presence of smokeemanating from a potential or incipient fire. Particle detectors of thescattering light type operate by exposing an air sample, that is drawnfrom an area being monitored, to light, and detecting light scatteredfrom any particles in the air. Air, for example from a factory oroffice, usually contains some level of particles, and the detector canbe set to alarm at certain levels which are higher than backgroundparticle levels, and are believed to be indicative of smoke.

The environments that scattering type smoke detectors operate in varywidely, and include, for example, office environments, factories andmanufacturing plant, power stations and clean rooms. Each of which hasdifferent levels of background particulate material.

A problem can be encountered with such smoke detection apparatus if theyare continuously exposed to the relatively high levels of backgroundpollution in the air that can exist in some environments. A large-scaleexample in recent years has been the high levels of smoke pollutionoften present in widespread regions of Asia, which have a highdependency on the burning of brown coal.

Background smoke pollution can cause soiling of components of thedetector leading to premature failure, for example due to clogging ofair paths or changes in the optical properties of the detection chamberitself.

Attempts to overcome this problem have included dust filters placed inthe airstream. Dust filters have been used to filter out particles notassociated with the smoke to be detected. Smoke particles may occur in avariety of sizes depending on the fuel used and combustion conditions,and the filter type is chosen according to type of dust particlesexpected and the type of smoke to be detected.

However, as conventional dust filters clog they begin to remove moreparticles from the air and will eventually begin filtering out smokeparticles (or other small particles of interest). This may be due toeffective pore size of the filter being reduced as more particles clogthe filter. This can be a problem because such filters start undesirablyremoving smoke particles before the flow rate changes appreciably. Theresult is that the filter may be removing an unknown proportion ofsmoke, but this is not detectable using flow-meters.

In some situations attempts have been made to condition the air sampleprior to its introduction into the smoke detector e.g. by diluting thesample flow with clean air. The object of such dilution is to arrive ata sample flow with an unchanged particle distribution, but with a lowerparticle concentration than the original sample flow.

Dilution can be used to effectively reduce the concentration ofparticulate material reaching the detector, but presents problems forair sampling smoke detectors that use a pipe network to draw air from aspace being monitored, in that the introduction of the diluent air flowinto the flow entering the detector reduces the amount of sample airdrawn from the region being monitored. This causes an increase in thetime taken for the sample air to travel from the region being monitoredto the smoke detector, and consequently increases detection time.

One proposed dilution filter, described in U.S. Pat. No. 5,332,512 toWells splits the sample flow into two sub-flows, and filters one of theflows to remove all particles from it. The filtered and unfilteredsample flows are then recombined.

The present inventors have ascertained that such a device would addressthe transport time increase identified above without requiring asubstantial increase in aspirator power, however, the dilution ratio ofsuch a device would change over time making taking reliable particlemeasurement difficult. More importantly the inventors have identifiedthat the dilution ratio will increase as the capillary, through whichthe unfiltered air passes, clogs. Ultimately this may lead to noparticles passing through the filter arrangement, which is undesirable.

SUMMARY OF THE INVENTION

In a first aspect there is provided an apparatus in an airflow pathbefore a particle detector, wherein the apparatus removes asubstantially constant proportion of all sizes of airborne particlesfrom the airflow over time.

The apparatus may further include a flow splitting arrangement fordividing the airflow into at least a first sub-flow and a secondsub-flow, and a filtering arrangement for filtering the first sub-flow.

In some embodiments the filter arrangement preferably removessubstantially all particulate matter from the first sub-flow.

The filtering arrangement can include a HEPA filter and/or anelectrostatic filtering means.

The flow splitting arrangement preferably includes a plurality ofapertures through which the airflow passes to divide it into sub-flows.

The plurality of apertures formed in the flow splitting arrangement arepreferably substantially identical to each other.

In certain embodiments the relative proportion of the airflow split intoeach sub-flow corresponds to the proportion of the apertures formed inthe flow splitting arrangement configured to direct the airflow intoeach sub-flow.

Preferably the flow spitting device has an impedance to the passage ofthe sub-flows that is substantially greater than the flow impedancecaused by the filtering arrangement. In certain embodiments, in theevent that the filtering arrangement is clogged so that it removesparticles to be detected by the particle detector to by unacceptableextent, the flow impedance of the flow splitting arrangement flow issubstantially greater than the flow impedance caused by the filteringarrangement.

In some embodiments the apparatus further includes, at least one flowmeter for determining a flow rate in any of the following:

an inlet to the apparatus;

an outlet from the apparatus;

a flow path through which one or more of the sub-flows passes.

The apparatus may include a plurality of flow meters.

In a second aspect the present invention provides a method of filteringan air sample prior to introduction to a particle detection means, themethod including: dividing the air sample into at least two sampleflows; filtering one or more of the sample flows; restricting the flowof the sample flows by an amount greater than a flow restriction causedby the filtering of the one or more sample flows; and re-combining atleast some of the sample flows prior to introduction to a particledetection means.

Preferably at least one sample flow is not filtered before beingre-combined with another sample flow.

The step of restricting the one or more filtered sample flows can beperformed either before or after the filtering of the sample flow.

The method can further include measuring any one or more of;

a flow of filtered air;

a flow of unfiltered air;

a sample flow;

the flow of air prior to dividing it into sample flows; and

the flow of air after combining sample flows compared to a total flow.

In a third aspect there is provided an apparatus for a smoke detectorincluding a first flow path having a filter and an aspirator, a secondflow path having an aspirator, and a controller, such that theaspirators adjust the flow in the first and second flow paths to providea predetermined ratio of filtered to unfiltered air.

In another aspect there is provided an arrangement for conditioning afluid flow, the arrangement including:

a first flow path;

a second flow path,

a filter arrangement to filter the fluid flow in the first flow path;and

a flow splitting arrangement for splitting the fluid flow into eitherthe first flow path or the second flow path,

wherein the impedance to the fluid flow caused by the flow splittingarrangement is greater than the impedance caused to the fluid flow bythe filter.

In yet another aspect the present invention provides an arrangement forconditioning a fluid flow including:

a chamber including a fluid inlet and a fluid outlet;

an unfiltered fluid flow path extending between the fluid inlet and thefluid outlet;

a filtered fluid flow path extending between the fluid inlet and thefluid outlet;

a filtering means for filtering the fluid flowing through the filteredfluid flow path;

a flow splitting arrangement for splitting the fluid flow into thefiltered fluid flow path or the unfiltered fluid flow path,

wherein the impedance to the fluid flow caused by the flow splittingarrangement is greater than that caused by the filtering means.

The flow splitting arrangement can include at least one first apertureleading to the filtered fluid flow path and at least one second apertureleading to the unfiltered fluid flow path, and wherein the flowimpedance caused by each first and second aperture is substantially thesame. Preferably, the proportion of the fluid flow split into thefiltered fluid flow path and unfiltered fluid flow path respectively isdetermined by the relative number of first and second apertures.

In a further aspect, the present invention provides a flow splittingarrangement configured to divide a fluid flow into a plurality ofsub-flows, the splitting arrangement including means for defining aplurality of substantially identically dimensioned flow aperturesconfigured to direct a portion of the fluid into a respective sub-flow,wherein the relative proportion of the fluid flow that is split intoeach sub-flow is determined by the relative proportion of theidentically dimensioned flow apertures configured to direct a portion ofthe air into each respective sub-flow.

Preferably, the flow means for defining a plurality of substantiallyidentically dimensioned flow apertures is a body having a plurality ofsubstantially identical apertures formed therein.

The body can be a plate-like member having a plurality of holes of equaldiameter extending through it to define said flow apertures.

In another aspect, the present invention provides a method of detectingparticles in an air sample, including:

(a) obtaining an air sample;

(b) reducing the concentration of particles in the air sample;

(c) detecting a level of particles in the air sample with reducedparticle concentration;

(d) applying a correction to the detected level of particlescorresponding to the reduction in concentration of particles in the airsample produced in step (b).

Step (b) may further include, splitting the air sample into sub flows;

-   -   filtering less than all of the sub-flows; and re-combining at        least one filtered and one unfiltered sub-flow to generate an        air sample with a reduced concentration of particles.

In another aspect the present invention provides an apparatus for asmoke detector including a first flow path having a filter and anaspirator, a second flow path having an aspirator, and a controller,such that the aspirators adjust the flow in the first and second flowpaths to provide a predetermined ratio of filter to unfiltered air.

Embodiments of this aspect of the invention have the advantage that theratio of filtered air to unfiltered air can be adjusted or kept constantwhen the impedance of the filtered flow path changes.

BRIEF DESCRIPTION OF THE DRAWINGS

An illustrative example configuration for such a device is described, byway of non-limiting example only, with reference to the accompanyingdrawings in which:

FIG. 1 is a schematic cross section of an example of a smoke attenuator;

FIG. 2 is a schematic top view of a perforated plate of the smokeattenuator of FIG. 1;

FIG. 3 is a schematic cross section of a filter of the smoke attenuatorof FIG. 1;

FIG. 4 is a perspective view of the filter of FIG. 3;

FIG. 5 is a schematic view of a first example of a smoke detector systemincluding the smoke attenuator of FIG. 1;

FIG. 6 is a schematic view of a second example of a smoke detectorsystem including the smoke detector of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 filtering apparatus 10, hereinafter termed a “smokeattenuator” is shown, having an inlet 12, an outlet 14, and a housing16. Within the housing 16 is a flow separator 18 (shown in FIG. 2) and afilter 20 (shown in FIGS. 1, 2 and 3). In the present example the flowseparator 18 is a plate 21 having a number of apertures 22 and 23 formedin it. These apertures are shown in greater detail in FIG. 2. In thisexample the flow separator separates the single flow of air entering theinlet into a filtered sub-flow (air passing through apertures 22) and anunfiltered sub-flow (air passing through aperture 23). The filtered andun-filtered sub-flows recombine in the area after the filter, beforeexiting the filter outlet 14. A flow distributor may be placed near theinlet 12 to assist in distributing the flow evenly within the housing.An insect screen 17, e.g. formed from a wire mesh, may also be placedwithin the housing to prevent insects and very large particulatematerial from contaminating the filter 20, or from passing via theun-filtered flow path into downstream components, such as a particledetector.

In alternative embodiments the sub-flows can remain separate for as longas desired so long as they are re-combined before entry into theparticle detection means.

In the present example, the smoke attenuator is designed to reduce smokeconcentration by a factor of 10. To achieve such a result, one tenth ofthe airflow is separated into a sub-flow by being directed through anaperture that introduces the sub-flow into a flow path that bypasses thefilter, and nine tenths are directed through apertures that require theairflow to pass through a filter before exiting the housing. In theparticular example shown in FIGS. 1 to 4, the filter is a highefficiency low impedance filter, such as a HEPA filter. In analternative embodiment, the filter could be an electrostatic filter.

Theoretical airflows are shown for illustrative purposes in FIG. 1. Inthis example, the sub-flow passing through the nine apertures in theouter ring of the plate 21 pass through the filter 20, which removessubstantially all particulate matter, be it smoke or dust. The sub-flowpassing through the single aperture 23 in the centre of the plate doesnot pass through the filter, and therefore retains substantially allparticles from the sample entrained in the air flow.

In the present case the filter has low impedance compared to the flowresistance caused by the apertures 22 and 23. As the filter clogs, itwill eventually increase the flow resistance to the air, decreasing theair flow through the filter and thereby increasing the proportion of theair that passes through the attenuator 10 without being filtered.

If the impedance of the attenuator is dominated by the flow restrictioncaused by the apertures, the ratio of filtered air to non-filtered aircan be made to change more slowly, effectively increasing the life ofthe filter. If the restriction caused by the apertures is much greaterthan that of the filter, the dilution ration can be kept substantiallyconstant over the effective life of the filter.

In a particularly preferred embodiment, the plurality of apertures 22and 23 in the flow splitter 18 are all the same size. Thus all of theapertures will tend clog at approximately the same rate, meaning thatover time as the apertures become constricted due to the build up ofcontaminants, the balance of flow between the unfiltered and filteredsub-flows will remain substantially constant.

Once the filter 20 clogs to a significant extent, it is possible tomeasure airflow changes either overall or specifically of the filteredair flow, to ascertain whether it is necessary to change the filter.

Another aspect of the present design is that as when the filter clogs toan extent that results in a change in the proportion of air flowingthrough the filter compared to air that bypasses the filter, theeffective smoke levels reaching the detector increase, meaning that if afilter is left in the system of the present invention too long it willcause an increase in the smoke detected, thus failing in a safe manner.This increase will be gradual as the system clogs, and may therefore bedetermined by software that checks the smoke levels over an extendedperiod of time to determine filter life.

Another method of determining filter life is to measure the airflow ofthe filtered air and to compare it to the airflow bypassing the filter.This ratio will give a smoke dilution factor that will allow the smokedetection system to apply a correction factor to determine true smokelevels in a sample. If the filter clogs over time, affecting flow ratesthrough the filter, the air flow meters will determine the newcorrection factor to be applied to the output of the detection chamber.In some embodiments it would also be possible to measure filter life bydetermining when the airflow through the filter had dropped to apredetermined level.

As noted above a feature of a preferred embodiment of the presentinvention is that all apertures in the flow splitter 18 are the samesize. One benefit of this arrangement is that airflow through theapertures can cause material to settle around the aperture periphery.Over time this will eventually reduce airflow significantly. Aspiratedsmoke detectors have a requirement that they pass sampled air at asufficient rate to the detector so that smoke levels can be determinedquickly, for example so that sampled air containing smoke levels abovethe preset level will trigger an alarm within one minute of the airbeing sampled. As the apertures become smaller, the airflow for a givenaspirator will drop, and eventually the time delay will exceed thespecifications. Aspirators are somewhat restricted in size and power dueto packaging constraints within the smoke detector system, and powerconstraints. For this reason aspirators are commonly fitted with flowsensors to determine whether the flow is above a predetermined level.

In the present example the apertures are 3.5 mm in diameter, whichprovides sufficient airflow through the smoke attenuator, and alsoprovides sufficient impedance over the impedance caused by the filter.

While the present example shows a flow splitter 18 having one aperture23 for unfiltered air and 9 apertures 22 for filtered air, there may beany number of apertures for filtered and unfiltered air flows providedthe ratio of apertures for the filtered sub-flow and the unfilteredsub-flow are known. This ratio should be provided to the detector sothat it can ascertain the dilution factor to apply to the air sampleentering the detector. For example, if the dilution factor is 10 (as inthe example in FIG. 1) then if the detector is to alarm when smokelevels exceed an obscuration of 1% per meter, then the detector willneed to alarm when the air outlet from the smoke attenuator exceeds 0.1%per meter. Aspirated smoke detectors such as the Vesda LaserPLUSmanufactured and sold by Vision fire & Security Pty Ltd, can easilydetect obscurations of 0.1% and lower.

As discussed above, the life of an aspirated smoke detector sampling aircapable of causing a constant obscuration of 1% per meter, can besignificantly lower than the life of an aspirated detector sampling aircapable of causing an obscuration of at 0.1% per meter, due to failuremodes such as blocking of detector flow paths, contamination of thesampling chamber causing an increasingly higher background level oflight etc.

FIGS. 3 and 4 show one example of a filter element that can be used inan embodiment of the present invention. Such filter elements have a highsurface area per volume and allow air to flow with low impedance.Filters may have a pore size, which is an aperture size that preventsparticles larger than the pore passing through the filter, chosen toremove all particles above a given size, or be configured to effectivelyblock all particulate material. In some filters, such as a foam filter,the effective pore size may be smaller than the measurable pore size asthe particle may have to travel along a significant pathway to passthrough the filter material.

In the present example, the filter material removes substantially allparticles able to be measured by the detector. In practice there is nosuch device as a perfect filter, and given the physical constrains tothe filter, it may be necessary to use a filter that merely removes someparticles from the air, or is selectively lets through particles under acertain size.

It should be noted that in an alternative embodiment the flow splittingarrangement could be located after the filter. In this case, rather thansplitting the flows prior to filtering, the flow splitting arrangementcan operate by limiting the flow through the filter and through theunfiltered flow paths by restricting the amount of air flowing out ofthem.

Embodiments of the flow splitting arrangement described herein couldalso be used in other applications to divide a fluid flow into aplurality of sub-flows. Certain embodiments may be used in otherapplications where maintaining a balance between a plurality ofsub-flows is desirable. Two exemplary smoke detection systemsincorporating a smoke attenuator 10 according to an embodiment of thepresent invention are shown in FIGS. 5 and 6. The smoke detection systemof FIGS. 5 and 6 each include a smoke detector 38 configured to detectsmoke in an air sample taken from a monitored a region 31.

In FIG. 5 air from the region 31 is drawn to the detector 31 via a pipenetwork 30 by aspirator 32. All the air drawn from the region 31 passesthrough a smoke attenuator 10 made in accordance with an embodiment ofthe present invention. The attenuator 10 reduces the smoke level in thesample flow to, for example, one-tenth its original value. After leavingthe smoke attenuator 10, part of the conditioned sample flow passesthrough the main pipe 33 to the aspirator 32 and is dumped toatmosphere. A second portion of the air sample is drawn along samplepipe 34 and through a detection chamber 38, such as a LaserPLUS chamberby a second aspirator 35.

FIG. 6 is similar to FIG. 5 except only the portion of the sample flowthat enters the detection chamber 38 passes through the attenuator 10,with the portion of the air that is exhausted to atmosphere withoutundergoing analysis remaining unfiltered.

The smoke attenuator 10 may also be used with a known smoke detectorsuch as a VESDA air sampling smoke detector, produced by Vision Fire &Security Pty Ltd, which operates with a single aspirator across thedetection chamber. It is not important from a detection perspectivewhether the smoke attenuator is placed in the full flow of the airthrough the main pipe, or if the smoke attenuator only filters the airin the sample pipe, The smoke attenuator can also be used in full flowsystems where all of the air drawn through the main pipe passes througha detection chamber.

In another example (not shown) there may be an aspirator in each flowpath to assist or cause flow through the flow paths. An aspirator may belocated in a filtered flow path, and another aspirator in an unfilteredflow path. A flow sensor may be in each flow path as well, so that acontroller can determine the flow ratios in each flow path, andtherefore ascertain the ratio of filtered air to unfiltered air. Thisarrangement would allow flow rates through the flow paths to bemonitored and adjusted to produce a desired outcome. One desired outcomemay be to keep the ratio of filtered air to unfiltered at a constantlevel.

The air sampled in the flow paths may be the full flow through samplingpipes, or may be a sub-sample of the air through the sampling pipes.Typically a sub-sample is used where a main aspirator draw air throughthe sampling pipes, which can be adjusted to produce an appropriate airtransit time. Aspirated smoke detectors are required to alarm within acertain elapsed time of the smoke being drawn into the sampling point.It is therefore necessary to maintain a sufficient flow rate through thesampling pipes to attain the transport time necessary to detect smokewithin the time limits. If variable speed aspirators are used in theflow paths of the smoke attenuator, then it may be necessary to use asub-sample arrangement.

In the above example, it is possible to use a controller to vary therate of flow in each flow path to achieve a particular smoke level atthe detector. For example, if background particles levels are at 1% permeter obscuration, and the detector is able to accurately measure smokelevels lower than 0.1%, then by detecting the smoke levels in the air,it is possible to vary the flow rate through each flow path to reducethe smoke level in the detector to the predetermined level. This has theadvantage that the life of the detector can be prolonged due to thereduced level of smoke in the detector flow paths, reducingcontamination of the detection chamber, apertures etc. In order todetermine if a threshold smoke level has been breached, the detectormonitors the smoke level in the detection chamber, and the flow rates ineach flow path, and can then determine the actual level of smoke in thesampling pipes.

As an example of the benefit of the smoke attenuator of the typedescribed herein, if 80% of the inlet flow passes through a “TotalFilter” (also known as a HEPA filter) while 20% remains unfiltered thenthe concentration of smoke will fall to one-fifth of the original.Consequently, the life expectancy of the detection chamber or thedetector itself may be increased fivefold over a detector that does nothave any filter, if clogging or background noise levels in the chamberare a failure mode. Of course, the fire alarm threshold(s) applied mustalso be adjusted to one-fifth of their ‘usual’ setting—but this is not aproblem in polluted environments, since the thresholds are normally atthe upper range of the detector's sensitivity. As such, while the smokeattenuator described herein is useful in many environments, it isparticularly useful in environments having high background smoke or dustlevels, and for use with sensitive detectors.

As can be seen from the above, the filter arrangement of the preferredembodiment operates by dividing an airflow into a number of sub-flows.In the illustrative embodiments described herein the airflow is splitinto two sub-flows, but it may be more. One or more of the sample flowsare filtered and the flows are re-combined to produce a conditionedflow. The flow rate (preferably relative flow-rate) of each sub-flows iscontrolled to control the relative proportion of the airflow enteringeach sub-flow. This will typically be performed by restricting thepassage of each sub-flow, either before or after filtering. Theresulting conditioned flow can then be used downstream, e.g. by beingprovided to a particle detector for analysis.

Whilst the exemplary devices for implementing this method that have beendescribed herein as a discrete filter arrangement in a housing, thepresent invention should not be considered as being limited to thisform. Alternatively, an embodiment may be implemented in using a networkor pipes or ducts, to separate the fluid flows, wherein the content ofsome, but not all pipes is filtered before recombination. In this casethe flow balance between the paths can be performed controlling the flowin each pipe or duct at any point between and including, the point ofdivision of the flows and the point of recombination of flows.

The reference to any prior art in this specification is not, and shouldnot be taken as, an acknowledgment or any form of suggestion that thatprior art forms part of the common general knowledge in Australia.

Throughout this specification, unless the context requires otherwise,the word “comprise”, or variations such as “comprises” or “comprising”,will be understood to imply the inclusion of a stated step or integer orgroup of steps or integers but not the exclusion of any other step orinteger or group of steps or integers.

The claims defining the invention:
 1. An apparatus in an airflow pathbefore a particle detector, wherein the apparatus removes asubstantially constant proportion of all sizes of airborne particlesfrom the airflow over time, the apparatus further including a flowsplitting arrangement for dividing the airflow into at least a firstsub-flow and a second sub-flow, and a filtering arrangement forfiltering the first sub-flow; and wherein the flow splitting device hasan impedance to the passage of the sub-flows that is substantiallygreater than the flow impedance caused by the filtering arrangement. 2.The apparatus as claimed in claim 1, wherein the filtering arrangementremoves substantially all particulate matter from the first sub-flow. 3.The apparatus as claimed in claim 2, wherein the filtering arrangementincludes a HEPA filter.
 4. The apparatus as claimed in claim 1 whereinthe flow splitting arrangement includes a plurality of apertures throughwhich the airflow passes to divide it into sub-flows.
 5. The apparatusas claimed in claim 4 wherein the plurality of apertures formed in theflow splitting arrangement are substantially identical to each other. 6.The apparatus as claimed in claim 4 wherein the relative proportion ofthe airflow split into each sub-flow corresponds to the proportion ofthe apertures formed in the flow splitting arrangement configured todirect the airflow into each sub-flow.
 7. The apparatus as claimed inclaim 1 that further includes, at least one flow meter for determining aflow rate in any of the following: an inlet to the apparatus; an outletfrom the apparatus; a flow path through which one or more of thesub-flows passes.
 8. The apparatus as claimed in claim 7, including aplurality of flow meters.
 9. An arrangement for conditioning a fluidflow, the arrangement including: a first flow path; a second flow path,a filter arrangement to filter the fluid flow in the first flow path;and a flow splitting arrangement for splitting the fluid flow intoeither the first flow path or the second flow path, wherein theimpedance to the fluid flow caused by the flow splitting arrangement isgreater than the impedance caused to the fluid flow by the filter. 10.An arrangement for conditioning a fluid flow including: a chamberincluding a fluid inlet and a fluid outlet; an unfiltered fluid flowpath extending between the fluid inlet and the fluid outlet; a filteredfluid flow path extending between the fluid inlet and the fluid outlet;a filtering means for filtering the fluid flowing through the filteredfluid flow path; a flow splitting arrangement for splitting the fluidflow into the filtered fluid flow path or the unfiltered fluid flowpath, wherein the impedance to the fluid flow caused by the flowsplitting arrangement is greater than that caused by the filteringmeans.
 11. The arrangement for conditioning a fluid flow as claimed inclaim 10 in which the flow splitting arrangement includes at least onefirst aperture leading to the filtered fluid flow path and at least onesecond aperture leading to the unfiltered fluid flow path, and whereinthe flow impedance caused by each first and second aperture is thesubstantially the same.
 12. The arrangement for conditioning a fluidflow as claimed in claim 11 wherein the proportion of the fluid flowsplit into the filtered fluid flow path and unfiltered fluid flow pathrespectively is determined by the relative number of first and secondapertures.
 13. A method of filtering an air sample prior to introductionto a particle detection means, the method including: dividing the airsample into at least two sample flows; filtering one or more of thesample flows; restricting the flow of the sample flows by an amountgreater than a flow restriction caused by the filtering of the one ormore sample flows; and re-combining at least some of the sample flowsprior to introduction to a particle detection means.
 14. The method asclaimed in claim 13 wherein the at least one sample flow is not filteredbefore being re-combined with another sample flow.
 15. The method asclaimed in claim 13 wherein step of restricting the one or more filteredsample flows is performed either before or after the filtering of thesample flow.
 16. The method as claimed in claim 13, further includingthe step of measuring any one or more of; a flow of filtered air; a flowof unfiltered air; a sample flow; the flow of air prior to dividing itinto sample flows; and the flow of air after combining sample flowscompared to a total flow.